Electromagnetic horn radiator



a) 6 Q) \JUKMUII nuu-u April 9, 1946. M. KATZlN 2,398,095

="" L4 ELECTROMAGNETIC HORN RADIATOR Filed Aug. 31, 1940 3 Sheets-Shet 1Qtml Ml mu 3 Sligets-Sheet 2 I l I III llll Ill IVENTQR v ATORNEY M.KATZIN ELECTROMAGNETIC P IORN RADIATOR -Filed Aug. 51. 1940 April 9,1946.

p i .9, 19 6. M. KATZIN 2,398,095

ELECTROMAGNETIC HORN RADIATOR Filed Aug. 51, 1940 3 Sheets-Sheet 3AT'ToiaNE Patented Apr. 9, 1946 team stai ELECTROMAGNETIC HORN RADIATORMartin Katzin, Riverhead, N. Y., assignor to Radio Corporation ofAmerica, a corporation of Delaware Application August 31, 1940, SerialNo. 354,954

8 Claims.

The present invention relates to ultra short wave antenna systems and,more particularly, to electro-magnetic horn structures and to means forefficiently coupling them to high frequency energy sources and/ortransducer means.

An object of the present invention is to increase the power gain ofelectro-magnetic horn antenna structures.

A further object is the provision of an electromagnetic horn structurehaving a minimum front to back length for a given frontal or mouth area.

Another object is the combination of a plurality of electro-magnetichorns to obtain large effective apertures while at the same timemaintaining a minimum over-all length.

A further object of the present invention is the combination of aplurality of electro-magnetic horn structures in such a way thatdistortions of the high frequency energy wave introduced by one horn arecompensated for by similar distortions of opposite phase in anotherhorn.

Still a further object of the present invention is to increase theefficiency of coupling between electro-magnetic horn radiators and theirexciting sources.

Still another object is to prevent the radiation of undesired waves fromhorn structures generated as a result of curvature of the horn orenergizing pipes.

A further object is the increase of efficiency of electro-magnetic hornradiating structures.

The present invention includes among its features the combination of aplurality of small horns of a predetermined ratio of length to frontalarea in such a way as to obtain a composite structure having a largefrontal area with a short over-all length. Furthermore, the horns thuscombined may be straight horns or they may be curved or folded in orderto still further decrease the over-all length of the structure.

Another feature of the present invention contemplates so designing therate of taper of the horn structures and their connection to theexciting means in such a Way as to obtain a maximum efiiciency ofcoupling between the horns and their exciting means.

Further objects, features and advantages of the present invention may bemore completely understood by reference to the following detaileddescription which is accompanied by drawings in which Figure 1 is anillustration of an electromagnetic radiating horn structure useful inexplaining the present invention; Figure 2 illustrates-a modification ofthe structure of Figure 1 in accordance with a feature of the presentinvention, while Figures 3, 4, 5 and 6 illustrate ways in which the hornstructures may be curved or folded in order to reduce their over-allfrontto-back length; Figures 7 and 8 illustrate improvements in the hornstructure whereby it is possible to obtain a desired amount of powergain with shorter over-all lengths, while Figures 9 and 10 illustratemodifications of a coupling pipe structure between the horn and itsexciting means whereby spurious or undesired frequency components areeliminated or reduced.

Figure 1 shows a, single horn antenna H having a width x, a height y andan over-all length Z. To the small, or throat, end of the horn iscoupled a short section of wave guide which acts as a resonant chamberand is indicated generally by reference numeral l2. Within the waveguide section I2 is located an exciting antenna [3 which is connected toa source of high frequency oscillations by means of transmission lineTL. The length of antenna I3 is preferably of the order of a quarter ofthe operating wavelength. The simplest wave, that is, the one with thelowest critical frequency for rectangular hollow pipe transmission whichgives a vertically polarized radiation in a single forwardly directedbeam, is known as an 110,1 wave. This wave has a component of magneticforce in the direction of propagation and a component of electric fieldintensity directed parallel to the y axis and at right angles to thedirection of propagation. There is no component of electric fieldintensity in the horizontal direction, i. e., along the x axis. Thesubscripts 0,1 denote the harmonic order of the wave in each coordinate.Since we are dealing, in the present case, with rectangular pipes onesubscript is required for each coordinate, the first subscript denotingthe number of half sine waves of electric field intensity along the a:axis and the second subscript thenumber of half sine waves of electricfield intensity parallel to the y axis.

Th antenna system shown in Figure 1 is designed to radiate verticallypolarized waves in a comparatively narrow beam, the sharpness of thebeam being determined by the frontal area m, y and the over-all lengthZ.

The horizontal dimension of the wave guide section I2 is chosen to beequal to, or greater than, a half of the operating free-spacewavelength. The vertical dimension is not critical, but with horns ofsquare aperture it is convenient to make it equal to the horizontaldimension.

The length of the wave guide section is at least long enough that astable condition of the radiant wave energy is established within thewave guide section before the energy arrives at the small end of thehorn. The distance between the exciting antenna l3 and the closed endwall I5 is on the order of a quarter wave-length. The exact distance isso chosen that the resultant impedance of the exciting antenna I3 isequal to the surge impedance of the transmission lin TL wherebyreflections of energ back along the transmission line TL to the excitingsource, not shown, are avoided. Since antenna systems of the type shownin Figure 1 must be mounted on tall towers in order to obtain as great atransmission range as possible, it is desirable to make the ratio of theover-all length to the frontal area as small as possible. One way inwhich this may be done is shown in Figure 2.

The single large horn structure ll of Figure 1 is, in Figure 2,substituted by a plurality of small horns 2|. Each of the small horns 2|may have a smaller ratio of length to frontal area than the single largehorn of Figure 1. The total over-all frontal area m, y 0f the radiatingstructure of Figure 2 is the same as the frontal area ac, y of Figure 1,while at the same time the length l, as can readily be seen, has beenshortened considerably. Each of the individual small horns 2| may beenergized by exciting antennas within the short wave guide sections l2,[2 in the same way as described with reference to Figure 1. The separateantennas of Figure 2 are connected through equal length transmissionline sections to the final transmission line TL which is energized fromthe exciting source. The radiation from all of the horns is therebyadditive in an in-phase relationship. In some cases it may be desirableto still further decrease the over-all length of the antenna systemshown in Figure 2, or it may be desirable to utilize a single large hornwith a small over-all length instead of a, plurality of small horns. Insuch cases the radiating horn structure, or structures, may beconstructed as shown in Figure 3.

The frontal area of the horn of Figure 3 is determined by the power gaindesired and the over-all length is decreased by folding the horn over ina vertical plane. The horn shown in Figure 3 is shown as having aconstant vertical dimension since this is a convenient form ofconstruction where vertically polarized waves are to be radiated, thoughit is also effective for horizontally polarized waves or for bothsimultaneously where different directivity patterns are desired. As maybe seen from the figure the horizontal dimension of the horn has avarying rate of taper. The advantages of this form of construction willbe discussed in more detail hereafter with reference to Figures 7 and 8.The exciting means, which is not shown in this figure but which issimilar to the wave guid and exciting antenna structure shown in Figures1 and 2, is connected to the throat or small end 33 of the horn in anydesirable fashion.

As in the case of Figure 1, an Ho,1 wave is applied to the horn havingonly a single electric field component parallel to the shorter sidewalls for waves of lowest critical frequency. An I-Ivo wave has itselectric field component parallel to the horizontal axis. The generaltype of H w ve in a r ctan ular wave guide system ma be designated as1-11, where m and n are positive integers, either one, but not both ofwhich may have the value zero.

Any H wave of a higher order than the H0, (or Hm,0) class has componentsof the electric field parallel to both the vertical and horizontal axes.Furthermore, all E type waves, that is, those having a component of theelectric field in the direction of energy propagation, also haveelectric field components parallel to both the long and short sidewalls. These properties ma be used to great advantage to control thetype of wave radiated by the horn. A grid 34 of horizontal wires isplaced across the opening of the mouth of the horn as shown in Figure 3.This grid is at right angles to the electric field component of thedesired H0,1 wave and, therefore, forms no obstruction to the radiationof the desired wave. However, other types of waves, as pointed outabove, have components of the electric field parallel to the wires ofgrid 34 and cannot, therefore, penetrate the grid 34. They are thusprevented from radiating whether the exist as a result of theirgeneration in the exciting tube structure, or as a result of therefiection of the Hon wave from the curved surfaces of the horn. Ifdesired, a smaller grid of horizontal wires may be placed before thebend in the horn, that is, at the small end 33 so that any E-wavesgenerated by the bend may be prevented from traveling back to thetransmitter as a reflection.

Other forms or folded horn structures are shown in Figures 4 and 5.These are similar to the one shown in Figure 3, except that the horn istapered in both vertical and horizontal directions. With appropriatedimensioning of the small end of the horn, it may be used either vn'thhorizontally, as well as vertically, polarized waves or both, asdesired. The grid 34 shown in Figure 3 may be employed with either ofthese horns when only a single polarization is used, orienting the grid,as explained above, to pass only the desired wave.

In Figure 4 the exciting means which may be similar to that shown inFigures 1 and 2, is connected to the small end of the horn 43. In somecases there may be insufficient space within the spiraled portion 44 forthe exciting structure so the spiral may be extended to one side asindicated by 54 in Figure 5.. This frees the small end 43 so that eitheran exciting means such as shown in Figure 1 may be readily connectedthereto even if it is of considerable length, or a long wave guide maybe connected thereto reaching to the transmitter location which may be,for example, at the base of the supporting tower structure for the horn.

Figure 6 illustrates a further modification which in one aspect may beconsidered as embodying two horn structures, such as shown in Figures 3or 4, arranged to be energized at 53 by a single energizing chamber. Thewave guide structure is split as indicated at 64 and bent around to formtwo separate radiating horns 5| and BI. In this form of construction itwill not, in all cases, be necessary to include a horizontal grating infront of the mouth of the horns 5| and BI in order to suppress thehigher order l-I waves and any E waves generated as a result ofreflections from the curved surfaces of the horns. Since waves emergingfrom the mouths of the horns travel an equal distance from the small end53, and the curvatures, being in opposite relative directions, generateE waves of opposite instantaneous polarities, as the E waves emerge fromthe mouths of the horns 5|. 6| they combine in opposing phaserelationship and cancel one another. While only two horns are shown inFigure 6 it is, of course, within the scope of the present invention touse banks of any multiple of two to obtain the desired frontal area.

In Figure 7 I have shown a modified form of horn which may be usedeither alone or in any of the combinations previously shown. In order toobtain the full power gain possible, it is necessary to reduce the rateof flare of a horn as the aperture of the horn is increased. Thus, whenhorn apertures are increased to obtain increased power gain, the lengthof the horn must be increased at a, greater rate. With linear taperingor flarin of the sides of the horn, the length of the horn must beincreased for two reasons as the frontal aperture is increased: first,because of the increase in aperture, and second, because of thedecreased rate of flare necessitated. It is therefore contemplated asshown in this modification to build the horn with a rate of flare whichis initially large but decreases constantly with increasin distance fromthe throat as required by the increasing aperture. In this way thedesired power gain may be obtained while the over-all length of the hornis less than would be the length of a horn of uniform taper. In Figure 7the horn H is indicated as flaring only in the horizontal plane, thedistance between walls 15 and 16 increasing at a more rapid rate nearerthe throat, but at a decreasing rate as they approach the mouth of thehorn. The walls 11 and 18 are indicated as being parallel since theparticular example illustrated is to be used only with waves of a singlevertical polarization. It is, of course, within the scope of theinvention to likewise taper the distance between walls 11 and 18 in thesame way as indicated for walls 15 and 16. While horns of square orrectangular cross section are preferred for linear polarization, it is,of course,

also within the scope of the present invention to utilize horns ofcircular or elliptical cross section, if desired.

Figure 8 shows a further modification of the form of the invention shownin Figure 7 in which a smoother transition from the exciting chamber [2is obtained by forming the horn with an increasing rate of flare nearthe throat as indicated by 85 and 86 of the side walls and then reducingthe rate of the flare as indicated by 15 and 16 in the way describedabove with reference to Figure 7. Thus reflections are avoided at thejunction of the throat of the horn to the exciting chamber l2, and stillthe advantages of the constantly decreasing rate of flare as describedwith reference to Figure 7 are obtained.

In the previously described embodiments of the present invention theexciting chamber l2 has been described as being quite short, that is,just long enough to insure that stable wave guide conditions areobtained before the wave guide is coupled to the radiating horn. In somecases it may be desirable to connect the radiating horn to thetransmitter by means of a wave guide or hollow pipe line a, number ofwavelengths long. In some cases the radiating horns may not present aperfect impedance match to the hollow pipe line even when using the hornstructure described with reference to Figure 8, so that some reflectionwill take place from the load end of the line, resulting in standingwave components in section [2 and, hence, affecting the frequencycharacteristic of the system for wide band operation. This may beovercome as shown in Figure 9 wherein a wave guide section 92 ofsubstantial length is coupled to the throat of horn II. The energizingstructure, which is not shown in this figure, is connected to end 93 ofthe wave guide 92. A branch section 94 of the hollow pipe line is joinedto wave guide 92 near the load end where the wave guide joins the throatof horn II. The far end of the branch section 94 is closed by a metalsheet 95 so that there is obtained the equivalent of a shunt branch lineshort-circuited at its far end. This gives the equivalent in hollow pipetransmission line technique to a shunt section of shortcircuitedtransmission line such as is used for impedance matching in the ordinarytwo conductor transmission lines. By locating branch section 94 at theappropriate distance from the throat of the horn, and making it theproper length, reflections will be eliminated along the wave guide 93from the branch section 94 to the transmitter. In the case of a curvedhorn this method of impedance matching may be used to serve a doublepurpose. In progressing around a curve in a hollow pipe through whichthere is transmitted an H type of wave, that is, a wave with nocomponent of electric field in the direction of propagation, someportion of the H wave energy is automatically converted into E waves,that is, waves possessing a component of the electric field in thedirection of propagation, by the bend in the hollow pipe line.

As shown in Figure 10, by placing the branch line 94 at the bend 96 ofthe wave guide 92 it is possible to equalize the E wave generated by thebend 96 in the main pipe line by an opposite E wave generated by thebend 91 in the branch line 94. The branch line thus is used for thedouble purpose of adjusting the termination of the feed line 92 for noreflection of the H wave which is being transmitted and for eliminatingany E waves generated by the bend in the line.

While I have particularly shown and described several modifications ofmy invention, it is to be distinctly understood that my invention is notlimited thereto but that improvements within the scope of the inventionmay be made.

I claim:

1. A horn radiator having rectangular mouth and throat apertures andbeing curved along its axis, means for energizing said horn with a waveof a polarization parallel to the plane in which the axis of said hornlies and means for suppressing radiant energy waves having a componentof polarization normal to said plane caused by the curvature of saidhorn.

2. A wave guide having rectangular mouth and throat apertures ofdifierent sizes and its axis being curbed in a vertical plane, means forenergizing said guide with a Wave having a component of electric fieldin a direction parallel to the axial plane of said guide and normal tothe direction of travel of said wave within said guide and means forsuppressing radiant energy waves having a component of electric fieldnormal to said plane caused by the curvature of said guide.

3. A wave guide having rectangular mouth and throat apertures and beingcurved along its longitudinal axis, means for energizing said guide atits throat aperture with a wave having a component of electric field ina direction parallel to the plane in which the axis of said guide liesand normal to the direction of travel of said wave within said guide andmeans for suppressing radiant energy waves having a component ofelectric field normal to said plane caused by the curvature of saidguide, said guide being tapered in a plane normal to the direction ofthe electric field component of said wave.

4. A tapered wave guide having rectangular mouth and throat aperturesand being curved along its longitudinal axis, means for energizing saidguide at its throat aperture with a wave having a component of electricfield in a direction parallel to the plane in which the axis of saidguide lies and normal to the direction of travel of said wave withinsaid guide and means for suppressing radiant energy waves having acomponent of electric field normal to said plane caused by the curvatureof said guide, said means comprising a grid transverse to the axis ofsaid guide and conductive only in a direction normal to the direction ofthe component of electric field of said wave.

5. A horn radiator having rectangular mouth and throat apertures andbeing curved along its axis, means for energizing said horn with a waveof a polarization parallel to the plane in which the axis of said hornlies and means for suppressing radiant energy waves having a componentof polarization normal to said plane caused by the curvature of saidhorn, said means comprising a grid transverse to the axis of said hornand conductive only in a direction normal to the direction ofpolarization of said wave.

6, A tapered wave guide having rectangular mouth and throat aperturesand being curved along its longitudinal axis, means for energizing saidguide at its throat aperture with a wave having a component of electricfield in a direction parallel to the plane in which the axis of saidguide lies and normal to the direction of travel of said wave withinsaid guide and means for suppressing radiant energy waves having acomponent of electric field normal to said plane caused by the curvatureof said guide, said means comprising a grid across the mouth of saidguide and conductive only in a direction normal to the direction of thecomponent of electric field of said wave.

'7. An electro-magnetic horn radiator comprising a plurality of curvedtapered horns and means for supplying wave energy of desiredcharacteristics to all of said horns in such relationship that theenergy from all of said horns is additive in a predetermined direction,said horns being arranged in pairs with the curvatures of the horns ofeach pair being in opposite directions whereby waves of an undesiredcharacteristic caused by said curvature are neutralized.

8. A wave guide having rectangular mouth and throat apertures and beingcurved along its longitudinal axis, means for energizing said guide atits throat aperture with a wave having a component of electric field ina direction parallel to the plane in which the axis of said guide liesand normal to the direction of travel of said wave within said guide andmeans for suppressing radiant energy waves having a component ofelectric field normal to said plane caused by the curvature of saidguide, said means including a branch guide connected to said firstmentioned guide and curved in the opposite direction along itslongitudinal axis, said branch guide being closed at its free end andbeing of such length that energy reflected from said closed end combineswith energy in the said first mentioned guide so that radiant energywaves having a component of electric field normal to said plane combinein a phase opposing relationship.

MARTIN KATZIN.

